Tasting Light and Remembering Bitter

Smelling bitterness, hearing coldness, feeling color, seeing pitch, and tasting light. All of these sensations fall under an umbrella of synesthesia. However, one may  have a certain sensation that could trigger a memory or response from a different sense. In a study conducted by researchers at MIT, the worm Caenorhabditis elegans was discovered to have the ability to detect light through its ability to taste hydrogen peroxide. Tasting hydrogen peroxide alerts the worm to other reactive oxygen compounds and allows it to have a better sense of awareness in its environment. All modes of light detection were formerly known to involve a chromophore, however, this worm may lead to the discovery of light sensitivity in a whole new realm. In this case, this adaptive light sensitivity through taste serves as a defense mechanism towards harmful substances in the environment. Researchers found that the worms stop eating when they are exposed to hydrogen peroxide and when they are exposed to light, suggesting that both light and hydrogen peroxide induce the same behavior. In the worm’s nervous system, certain receptors (such as LITE-1) within a neuron are responsive to the level of hydrogen peroxide as well as have a level of light sensitivity.  This suggested that certain receptors detect light indirectly by first “detecting reactive oxygen compounds generated by light” (2). While this group of receptors is specific to invertebrates, humans have an enzyme called peroxiredoxin that has the potential to play a role in oxygen detection by light in the human eye.

Much like how the MIT study describes an association with gustatory and light perception, a recent study by Alex C. Keene and Pave Masek of the Mac Planck Institute of Neurobiology delves deeper into the relationship between optics and gustatory behavior. In particular, their study highlighted how optics or optogenetics have the ability to activate or suppress taste behavior through memory. By exposing Drosophila to various laser beams at different regions on their bonding and by observing this influence on certain subpopulations of taste neurons which then induced a certain behavior, researchers were able to better understand the neural circuitry that defines behavior. The researchers found that after using IR-lasers to activate bitter-sensing neurons, the flies had a similar suppression response when exposed to natural quinine. It’s as if they had been conditioned towards remembering a bitter response through the use of lasers. This discovery is comparable to the first study in that while both light and hydrogen peroxide cause C. elegans to suppress food consumption, both laser activated bitter sensations and naturally bitter substances induce Drosophila to suppress their consumption as well. Both of these studies draw a connection between what was once separated as optical perception and gustatory sensation, tiptoeing on the line of what could trigger synesthesia and suggesting a possibility that synesthesia could be conditioned into organisms over time.

Keene, A. C., Masek, P., (2012). Optogenetic induction of aversive taste memory.

            Neuroscience,  222, pages 173-180.

Trafton, A. (2015, January 29). Tasting light. Retrieved May 1, 2015, from

http://newsoffice.mit.edu/2015/hydrogen-peroxide-taste-receptors-sense-light-0129